WASHINGTON—Tightened border security is at the center of immigration proposals in Congress, and for many lawmakers that means greater use of drones and other high-tech monitoring equipment.

But tryouts of drones and blimps along U.S. borders suggest the aircraft are more expensive and complex to operate than the government expected.

The U.S. Customs and Border Protection, part of the Department of Homeland Security, used its drones just over one-third of the time they were available, owing to shortages of qualified staff, flight limitations imposed by regulators and other issues, according to a May 2012 report by Homeland Security's inspector general. The border agency has used unmanned planes for nearly a decade.

Mike Friel, spokesman for the border agency, said the drones are "only one of the tools we use to accomplish our border security mission." He added that thanks to effective border surveillance, apprehension of people trying to cross the border "remains at historic lows."

Backers of drones say border agents would benefit from additional technology that proved useful to the U.S. military.

"In Pakistan and Afghanistan, these things have been hugely successful in crippling al Qaeda and the Taliban, really decimating their command-and-control structure," said Rep. Michael McCaul (R., Texas), the chairman of the House Homeland Security Committee. Border drones will be "visibility assets," he said.

Unlike drones in war zones, used by the military for both intelligence gathering and lethal airstrikes, the DHS's domestic drone fleet isn't armed. The border drones can fly as long as 20 hours and as high as 50,000 feet, along the southern and northern borders and in the Caribbean. The planes use cameras and radar to interact with on-the-ground sensors and surveillance towers to keep watch on vast stretches of border that are often difficult to access. The 10 unmanned plane systems each cost about $18 million.

Efforts to beef up border security are tied to the bipartisan Senate proposal that could provide legal status to some of the estimated 11 million undocumented immigrants living in the U.S. The proposal requires the Obama administration to certify to Congress security standards before allowing immigrants to qualify for citizenship, and it calls for more unmanned aerial vehicles along the border. Lawmakers are negotiating how to define those security standards.

Mr. McCaul said he was preparing legislation to require the Obama administration to use high-tech military equipment being returned from the wars as part of a broader security strategy, with the goal of providing metrics to determine how secure the border is. That goal is in line with the bipartisan immigration overhaul now being proposed.

Complicating any proposed technology fixes: Homeland Security's past troubles with technology projects, such as its failed billion-dollar "electronic fence" system. The system of radar and camera towers was supposed to give agents mobile views of stretches of rugged border terrain, but it never worked as planned. The department canceled the program in 2011.

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Mr. McCaul says the project called SBInet, which included the virtual-fence plan, set back the cause of border security five years. Meanwhile, he says, better security elsewhere along the 2,000-mile U.S.-Mexico border has put a greater burden on his home state. "There are more crossings in Texas because of towers built in Arizona," he said.

A Homeland Security official cautioned against lawmakers prescribing specific technologies, saying the agency needs flexibility to respond when smugglers change tactics and crossing points. The official said the agency doesn't believe it needs surveillance that provides "100% coverage 24/7" along the border, but rather prefers to use technology to detect changes in patterns among smugglers.

Homeland Security last year began testing blimps equipped with cameras and radar, similar to those used in the overseas wars, in parts of the border.

The review by the inspector general concluded Customs and Border Protection had "shortfalls of qualified staff and equipment," which, together with flight limitations imposed by the Federal Aviation Administration, limited actual flight time. Without enough personnel and equipment to operate the drones, the review found, the border agency had to take $25 million from other programs to plug gaps in the drone operations during the 2010 budget year. The agency has agreed with the report's recommendations.

Meanwhile, the prospect of increasing the drone fleet is prompting some unease among civil libertarians because the border agency allows local law enforcement and federal agencies to use the aircraft for special missions over U.S. territory.

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